Many biodegradable polymers used in medical articles degrade through hydrolytic mechanisms. Common examples of such biodegradable polymers are poly(hydroxy acids) such as polylactide, polyglycolide, poly(lactide-co-glycolide) and polycarprolactone, polyanhydrides, and polyarylates, among others. The degradation byproducts produced by such polymers are frequently acidic, which may create a low pH environment in and around the polymer and result in an inflammatory response.
Acidic degradation byproducts may also have a profound effect on how the devices degrade. For example, it is known that polylactide, polyglycolide, poly(lactide-co-glycolide) display self-catalytic activity when they degrade in aqueous environments. In this regard, it is believed these polymers degrade in the bulk such that degradation byproducts are entrapped, thereby creating a low pH environment, which causes the biodegradation of the polymers to accelerate. This phenomenon can cause the devices to degrade into an empty shell under certain conditions, particularly where the device is sufficiently thick and has sufficient water permeability. Unfortunately, this type of degradation is an inferior degradation mechanism for certain applications, such as biodegradable stents. To prevent fragmentation of medical articles at a late stage of use, it is typically preferred that surface erosion be the prime mechanism for degradation.
Furthermore, in applications where therapeutic and diagnostic agents are included within matrices formed using biodegradable polymers, low pH environment may also be of concern if the agents are sensitive to acidic conditions (e.g., where a low pH environment may cause structural changes to the agents).